This invention relates to tensioners used with chain drives in automotive timing applications and more particularly to a blade-type chain tensioner in which two shoes overlap each other.
Tensioning devices are used as a control device for a power transmission chain as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises or slippage. Prevention of slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage may alter the camshaft timing by several degrees, possibly causing damage. In the harsh environment in which an internal combustion engine operates, chain tension can vary between excessively high or low levels as a result of the wide variations in temperature and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. Camshaft and crankshaft induced torsional vibrations cause chain tension to vary considerably. This tension variation results in chain elongation. Moreover, wear of the chain components during prolonged use can cause elongation of the chain that results in a decrease in the tension of the chain.
Generally, blade-type chain tensioners use a blade spring interlocked under tension with a single shoe to provide tension to a chain. The blade spring is arcuate in shape and the shoe is relatively flat. The shoe is constructed from a semi-rigid material which will deform or "creep" upon experiencing a load at a high temperature. The blade spring is flattened to correspond to the shape of the shoe and then interlocked with it. Because the semi-rigid shoe prevents the blade spring from returning to its original more arcuate shape, the blade spring applies a load to the shoe. During operation, as the heat from the engine causes the temperature of the shoe to increase and become less rigid, the load from the blade spring causes the shoe to deform to a more arcuate shape. Through such deformation, tension is provided to a chain. The chain tensioner assembly is positioned along a free length of the chain between the sprocket gears. As the blade spring forces the shoe into a more arcuate shape, the apex of the shoe extends farther into the span of chain thereby increasing chain tension.
Typical blade-type chain tensioners have interlocked a blade spring to only a single shoe. For example, U.S. Pat. No. 3,490,302, to Turner et al., discloses a chain tensioner where the blade spring is mounted to mechanically interlock with, and thereby provide a load to, a shoe through a hole and pin combination. The blade spring continuously bears against the shoe.
Another structure for mounting a blade spring to a single shoe is disclosed in U.S. Pat. No. 4,921,472, to Young et al. This reference discloses a blade-type tensioner having a blade spring mechanically interlocked with a shoe through a passageway in the end of the shoe without the use of a pin.
Yet another structure for mounting a blade spring to a single shoe is disclosed in U.S. Pat. No. 5,055,088, to Cradduck et al. This reference discloses a blade-type tensioner which utilizes a plurality of blade springs interlocked with a single plastic shoe through a passageway in the shoe and fastened using a pin.
An improved construction for mounting a blade spring to a shoe is disclosed in U.S. Pat. No. 5,266,066, to White, incorporated herein by reference. This reference discloses a blade-type chain tensioner in which a blade spring is constructed from a simple rectangular metal band formed into an arcuate shape and interlocked with a pocket in a shoe to provide a load to the shoe.
Unfortunately, the prior art blade-type chain tensioners have certain drawbacks. For one, the range of the arcuate shape of the shoe is limited. With timing chains that have longer sprocket gear center distances, as the chain temporarily lengthens due to varying operating temperature and tension variation, the chain achieves greater vertical movement compared to a chain having a shorter center distance. Also, the chain may permanently lengthen due to wear and permit increased vertical movement of the chain between the sprocket gear centers. To limit this vertical movement and take up this additional slack, the chain tensioner would have to bend to a more arcuate shape. However, the prior art chain tensioners may not be able to bend enough to meet the chain with adequate force to maintain the proper tension on the chain because the prior art tensioners resist great changes in shape. Usually, a much stronger blade spring is required to place a higher initial pre-load on the shoe to force it to the more arcuate shape necessary to take up the additional slack in the chain when it has aged and lengthened. Unfortunately, the stronger blade spring may cause the chain tensioner to impart too high a tension on a new chain during initial operation.
Another drawback of the prior art blade-type chain tensioners is that they are prone to prolonging oscillation of the chain. As discussed above, the harsh operating conditions and torsional vibrations of the engine induces varying tension in the chain. The blade spring reacts to the varying tension in the chain imparted by the torsional vibrations. Depending on the vibrational frequency, the spring force of the blade spring may react with a resonant vibration that establishes a prolonged oscillation of the chain. It is desirable to minimize this oscillation and maintain a constant tension on the chain.
The present invention provides a blade-type chain tensioner that addresses these limited arcuate range and oscillation problems. By providing two chain tensioner shoes in an overlapping configuration, one shoe imparts tension to the chain, but the other shoe damps the movement of the first shoe to reduce the oscillations. The overlapping shoe configuration also allows a greater range of arcuate movement so that the chain tensioner is able to meet the greater transverse movement associated with a longer center length timing chain. The present invention attempts to achieve greater arcuate movement with a lower initial blade spring tension.